Advances in radio technology have led to a demand for a higher level of circuit integration, with different circuits being integrated into a single system on a chip. For example, a radio frequency (RF) front end may include a power amplifier, a low noise amplifier and a switch. Integration of a power amplifier in complementary metal oxide silicon (CMOS) technology can enable a low cost but, for RF usage, such a power amplifier is required to sustain a high power output at a high frequency. Ideally, the power amplifier should combine high power with high efficiency, without efficiency being reduced when the amplifier is operated at low power. One known solution is based on the use of a high efficiency power transistor with power regulation, such as envelope tracking.
Referring to FIG. 1, a basic amplifier 10 without envelope tracking has a transistor 12 having a gate coupled to a signal source 11 by means of a first capacitor 13. A drain of the transistor 12 is coupled to a voltage supply node 14 at a constant supply voltage Vdd by means of an inductor 15. A load 17 is coupled to the drain of the transistor 12 by means of a second capacitor 16. FIG. 3, graph (a) illustrates an amplified signal at the drain of the transistor 12. Due to the inductor 15, the drain has a voltage swing between zero and about 2Vdd. In FIG. 3, the area between the amplified signal and the constant voltage 2Vdd represents power that is unused, not being delivered to the load 17, instead being dissipated as heat, thereby resulting in a low efficiency. Biasing of the gate of the transistor 12 is not illustrated in FIG. 3.
Referring to FIG. 2, an amplifier 20 including envelope tracking has the same components as the basic amplifier of FIG. 1, coupled in the same way, except that the voltage supply node 14 is at a non-constant supply voltage Venv instead of the constant supply voltage Vdd, and the amplifier 20 comprises an envelope tracking stage (ENV) 18 coupled between the signal source 11 and the voltage supply node 14. The envelope tracking stage 18 generates the non-constant supply voltage Venv which tracks the envelope of the signal delivered by the signal source 11, having a maximum value of Vdd. The non-constant supply voltage Venv may therefore be referred to as an envelope signal Venv. FIG. 3, graph (b) illustrates the amplified signal at the drain of the transistor 12 of the amplifier 20, which is unchanged from FIG. 3, graph (a) and, due to the inductor 15, has an amplitude of about 2Venv and can reach a maximum of about 2Vdd. FIG. 3, graph (b) also illustrates twice the non-constant supply voltage Venv. The only power that is dissipated is represented by the area between the amplified signal and twice the non-constant supply voltage Venv. It can be seen that compared with FIG. 3, graph (a), power efficiency is improved by the envelope tracking and heat dissipation is reduced.
There is a requirement for an improved amplifier and method of amplification.